That was my suspicion as well. This is outside my area of expertise, but from what I can tell the dosing isn't (multiple) orders of magnitude larger.

From the article, they mention about 4 times higher dosing than detected in humans when the MPs enter through medical supplies (including through surgery).

  - "about 12 μg of MPs [microplastics] per milliliter of blood have been detected in human blood. [prev studies mentioned above]"  (I haven't read those references, though, just going off a quick skim)
  - "We would like to bring mouse blood MPs to this level by injection."
  - "the diluted final concentration after entering the bloodstream should be blood of about 50 μg/mL" [sic]

A quick search for levels detected in humans led me to this paper [1] that gives 1.84 - 4.65 μg/mL, though with "a mean particle length of 127.99 ± 293.26 µm (7-3000 µm), and a mean particle width of 57.88 ± 88.89 µm (5-800 µm)." compared to uniform 5-μm-diameter microsphere used in the submitted article.

So the mouse dosing is (compared to humans):

  - 4 times higher than contamination through medical interventions (if i understand correctly)
  - 15 times higher than normal contamination  (only based on the one article)

So higher, for sure, but still rather close in cases with a lot of contamination. Not sure how the particle size factors into it

[1] Microplastics in human blood: Polymer types, concentrations and characterisation using μFTIR https://doi.org/10.1016/j.envint.2024.108751

EDIT: formatting and rephrasing